Gas turbine engines on a wide variety of commercial and military aircraft are started with air turbine starters. The pneumatically-driven starter applies acceleration torque to an input shaft of a gearbox which is in driving engagement with a tower shaft, and the tower shaft is in turn in driving engagement with the main shaft of the engine. From a cold start, the starter provides all acceleration torque until the engine reaches its light-off speed, after which both the starter and the engine provide acceleration torque until the engine reaches a predetermined speed commonly referred to as the "starter cut-out speed". At the cut-out speed, pneumatic power to the inlet of the starter is discontinued. Thus, prior turbine engine starting methods include the steps of sensing the rotational speed of the starter output shaft and responding to the cut-out speed by decreasing mass flow of air to the starter inlet.
During startup acceleration, driving engagement between the starter and the engine is effected through a clutch which couples the output shaft of the starter with the input shaft of the gearbox. When pneumatic power to the starter is discontinued, backdrive torque develops on the output shaft of the starter. This torque is used to assist disengagement in a manner which depends on the design of the clutch. Some starters incorporate an overrunning clutch that allows the output shaft to be driven by the engine without engaging the gear train of the starter. Others effect disengagement in a manner which permits the output shaft to come to rest. All pose mechanical problems that limit the ability to timely restart the engine when it is decelerating. Avoiding clutch failures and damage induced by high-speed running engagements requires that the engine be permitted to decelerate until its speed is within a range which is acceptable in view of the mechanical limitations of the starter. This invention is directed to overcoming the above-described limitation.